1. Field of the Invention
The present invention relates to a leaf spring for a suspension in cars such as passenger cars, trucks, buses, and trains, and the like, and relates to a production process for the same, and particularly relates to technologies to maximally improve the durability thereof.
2. Description of the Related Art
Heretofore, a leaf spring for a car (hereinafter referred to simply as a “leaf spring”) is produced, after forming a spring steel, by quenching, tempering, and performing a shotpeening at ordinary temperatures. The shotpeening in this case is a process in which shot made from steel are impacted at high speed on a surface, in which tensile stress occurs when the leaf spring is mounted in a car, thereby generating compressive residual stress in the surface portion and improving durability.
In recent years, a stress-peening in which shotpeening at ordinary temperatures is performed to impart stress to the spring steel is also known, as proposed in U.S. Pat. No. 959,801 and Japanese Patent Application, First Publication, No. 148537/93. In such stress-peening, a large residual compressive stress can be obtained compared to that in conventional shotpeening.
Spring steels for leaf springs, SUP6 (silicon manganese steel), SUP9 or SUP9A (manganese chrome steel) and SUP11A (manganese chromium boron steel) have been popular, and Brinell hardness thereof after heat treatment of hardening and tempering is 388 to 461 HBW (corresponding to a diameter of 2.85 to 3.10 mm on a Brinell ball mark). In recent years, research on the use of SUP10 (chromium vanadium steel) of which the Brinell hardness is 444 to 495 HBW (corresponding to a diameter of 2.75 to 2.90 mm on a Brinell ball mark). According to this steel type, since the hardness is high and the grain can be fine, the durability can be further improved, although the residual compressive stress is approximately equal to that in the case in which the stress-peening is performed.
FIG. 8 is an S-N diagram showing results of an endurance test using a leaf spring (1) which is the steel type of SUP9 or SUP9A, SUP11A and in which the shotpeening at ordinary temperature is performed after the heat treatment, a leaf spring (2) which is of the same steel type as the leaf spring (1), in which stress-peening at ordinary-temperature is performed after the heat treatment, and a leaf spring (3) which is of the steel type of SUP10 in which stress-peening is performed after the heat treatment. It should be noted that in this endurance test, the stress (mean stress) of 686 MPa was set in the leaf spring, and a stress amplitude was given to the stress. As shown in FIG. 8, the endurance frequencies were shown to be (1)<(2)<(3). Residual compressive stresses in the leaf springs (2) and (3) were 80 kgf/mm2.
Thus, in the case of performing the stress-peening by using SUP10, the durability is greatly improved. However, there is a disadvantage in that the material cost for SUP10 is high since it is more expensive than SUP6 and SUP9.